Method and device for checking an electrical value of an electric machine

ABSTRACT

The invention relates to a method for checking an electrical value of an electric machine, in particular an electric machine of a coordinate measuring device. The electric machine has an electric drive comprising a stator and a rotor. The method includes the steps of: detecting a value of a drive current delivered to the electric drive for driving the rotor, detecting a measured value of an electrical input variable of the electric machine, determining a calculated value of the electrical input variable on the basis of the detected value of the drive current and on the basis of a performance model of the electric machine, and determining a comparison value on the basis of the detected measured value and the calculated value of the electrical input variable, in order to check the detected value of the drive current.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of international patent application PCT/EP2016/061669 filed on May 24, 2016 designating the U.S., which international patent application claims priority from German patent application DE 10 2015 110 174.2 filed on Jun. 24, 2015. The entire contents of these priority applications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a method for checking an electrical variable of an electric machine, in particular an electric machine of a coordinate measuring machine, wherein the electric machine has an electric drive having a stator and a rotor.

The invention further relates to an apparatus for checking an electrical variable of an electric machine, in particular an electric machine of a coordinate measuring machine, wherein the electric machine has an electric drive having a stator and a rotor.

The present invention further relates to a method for operating an electric machine.

The present invention finally relates to an electric machine having an electric drive, which has a stator and a rotor, and having a control unit for actuating or energizing the electric drive.

It is generally known from the technical field of electric drives that the motor current as an electrical variable is proportional to a motor force for linear motors and to a motor torque for rotary motors. The relationship between said electrical variable and the mechanical variable is given by a drive-specific characteristic variable. For the precise and safe control of electric drives it is therefore necessary to reliably detect the motor current as a measurement variable for determining or for limiting motor forces or motor torques and to provide said motor current for the control of the motor. For reliable measurement of the motor current in this way it is usually necessary to redundantly detect the motor current, in order to achieve the appropriate level of safety.

A corresponding electric drive for a coordinate measuring machine, in which the speed and/or the acceleration of the rotor is controlled or limited on the basis of the detected motor current, is known, for example, from DE 10 2006 003 362 Al.

The motor current of DC motors is usually detected by a shunt resistor in the motor line or by a Hall sensor and can additionally be redundantly detected in the central controller, with the result that redundant motor current measurement is possible with little outlay.

The electrically commutated DC machines (BLDCs) often used today usually have three phases, which are offset with respect to one another by 120°, wherein the motor current is detected in at least two of the three phases, preferably, however, in all three phases, in order to determine or to limit the torque of the drive. In this case, however, a technically complex transformation of the three phase currents is required for determining the torque.

Since the detection of the effective motor current is technically complex and complicated in the case of the multi-phase electrically commutated DC machines, most drives of this kind do not involve redundant detection of the motor current, with the result that correspondingly reliable measurement is not possible for safety-relevant applications.

Furthermore, in the case of a decentralized arrangement of the control unit, the electric drive is usually controlled by means of a digital bus system. For safe motor control, however, the detected motor currents have to be reliably transmitted to the central controller by means of the digital bus system, with the result that the transmission of the measurement value of the motor current to the central controller is generally associated with a high technical outlay.

SUMMARY OF THE INVENTION

It is therefore the object of the present invention to provide confirmation of the detected motor current for an electric drive with little technical outlay.

In accordance with a first aspect of the present invention, a method for checking an electrical variable of an electric machine of the type mentioned at the outset is therefore provided, said method having the steps of detecting a value of a drive current, which is fed to the electric drive for the purpose of driving the rotor, detecting a measurement value of an electrical input variable of the electric machine, determining a calculation value of the electrical input variable on the basis of the detected value of the drive current and of a performance model of the electric machine, and determining a comparison value on the basis of the detected measurement value and of the determined calculation value of the electrical input variable, in order to check the detected value of the drive current.

In accordance with a second aspect of the present invention, an apparatus for checking an electrical variable of an electric machine of the type mentioned at the outset is therefore provided, said apparatus having a first detection unit, which is associated with the electric drive and is configured to detect a value of the drive current, which is fed to the electric drive for the purpose of driving the rotor, a second detection unit, which is configured to detect a measurement value of the electrical input variable of the electric machine, a determination unit, which is configured to determine a calculation value of the electrical input variable on the basis of the detected drive current and of a performance model of the electric machine, and a checking unit, which is configured to determine a comparison value on the basis of the detected measurement value and of the determined calculation value of the electrical input variable, in order to check the detected value of the drive current.

Furthermore, a method for operating an electric machine is provided, said method having the steps of checking an electrical variable of the electric machine in accordance with the present invention and interrupting an electrical power supply of the electric drive if the comparison value exceeds a predefined threshold value.

Finally, an electric machine of the type mentioned at the outset having an apparatus for checking an electrical variable of the electric machine in accordance with the present invention is provided.

In accordance with the invention, a performance model of the electric machine is understood to mean an observation of the consumed and output powers taking into account the conservation of energy.

In accordance with the present invention, an electrical input variable of the electric machine is understood to mean an electrical input voltage or an electric input current of the entire electric machine, wherein the electrical input variable forms at least one electrical variable of the electrical power consumed by the electric machine.

The drive current, which is fed to the electric drive for the purpose of driving the rotor, is detected by the method according to the invention and the apparatus according to the invention for checking an electrical variable of the electric machine and the electrical input variable is calculated on the basis of the detected drive current and an observation of the electrical power consumed by the electric machine, with the result that the measured drive current can be confirmed by comparing the calculated value of the electrical input variable with the measurement value of the electrical input variable. The drive current can be checked metrologically by way of said comparison of the measured electrical input variable and the electrical input variable calculated on the basis of the drive current, with the result that additional redundancy can be provided with little technical outlay and the reliability of the motor current measurement can be increased thereby. The drive current is detected with a certain factor of uncertainty by the method according to the invention and the apparatus according to the invention and can be checked by the theoretical performance observation and the measurement of the electrical input variable by a second measurement value, with the result that the measured value of the drive current can be confirmed with little technical outlay.

A safe and reliable advancing force limitation for linear drives and a safe and reliable drive torque limitation can be achieved by the thus confirmed measurement of the motor current, with the result that a sufficient level of safety for personal protection or the like can be provided with little technical outlay.

Finally, confirming the motor current by way of the theoretical performance observation and the electrical input variable of the electric machine that can be measured externally with little technical outlay can significantly reduce the technical outlay with respect to reliable transmission of the confirmed motor current, since the measurement value can be confirmed in the central control unit by way of the performance model.

As a result of the fact that, in the method according to the invention for operating an electric machine, the electrical power supply of the electric drive is interrupted if the comparison value exceeds a predefined threshold value, an erroneous measurement of the drive current can be identified and the electric drive can be reliably switched off when the limitation of the advancing force or the limitation of the torque can no longer be reliably ensured. The reliable function and the safety of the electric machine can therefore be significantly increased.

The object of the present invention is therefore achieved in full.

It goes without saying that the preferred refinements of the method likewise form refinements of the apparatus or can be applied thereto.

In a preferred refinement, the electrical input variable is a total current consumed by the electric machine or an electrical variable corresponding to the total current consumed by the electric machine.

As a result, it is possible to detect an electrical variable that varies depending on the electrical power consumption of the electric machine with little technical outlay.

In this case, it is particularly preferable if an electrical voltage is detected at a measurement resistor of an electrical input line of the electric machine, in order to measure the total current consumed by the electric machine.

As a result, the total current consumed by the electric machine can be detected with little technical outlay.

It is furthermore preferable if the calculation value of the electrical input variable is calculated on the basis of a total voltage supplied to the electric machine.

As a result, the total electrical power consumed by the electric machine can be determined precisely.

In a preferred refinement, the performance model takes an electromotive force or an electromagnetic counterforce of the electric drive into account.

As a result, the electrical power consumed by the electric drive and not converted to movement energy can be taken into account, as a result of which the performance observation by means of the performance model is particularly precisely possible.

In this case, it is particularly preferable if a movement variable of the rotor is detected and the electromotive force or the electromagnetic counterforce is determined on the basis of a characteristic variable of the electric drive and of the detected movement variable.

As a result, the electromotive force or the electromagnetic counterforce can be determined independently of the electrical input variables.

In a particular refinement, the performance model takes an electrical power loss of a control unit of the electric machine into account.

As a result, additional electrical loads of the electric machine can be taken into account in the performance model, as a result of which the performance observation is more precisely possible.

In this case, it is particularly preferable if the electrical power loss of the control unit is determined when the rotor is stationary.

The electrical power loss of the control unit when the rotor is stationary is in this case determined by means of the power consumption when the rotor is stationary, wherein the electrical power consumed in moving operation is determined on the basis of the supply voltage of the electric machine and the power consumption is determined when the drive is stationary. As a result, the power consumption of the control unit can be reliably determined with little technical outlay.

It is furthermore preferable if the performance model takes a mechanical drive power of the electric drive into account. The mechanical drive power preferably results here from the effective drive current, which is consumed by the rotor of the electric drive and the electrical resistor of the rotor, which is given or can be determined in advance as the motor constant.

As a result, the mechanical drive power as a component of the performance model can be determined on the basis of the measured drive current with little technical outlay.

In a preferred refinement, the comparison value is determined on the basis of a difference value between the measurement value and the calculation value of the electrical input variable.

As a result, a deviation of the detected value of the drive current from an expected value of the drive current can be determined with little technical outlay, with the result that an erroneous measurement can be identified with little technical outlay.

It is furthermore preferable if the comparison value is determined on the basis of a sum of a plurality of the difference values.

As a result, smaller temporary differences between the measured drive current and the calculated comparison value can be balanced by the corresponding mean value formation, with the result that temporary differences do not lead to the electric machine being switched off.

It is furthermore preferable if the comparison value is determined on the basis of an integral of the difference value.

As a result, a continuous, precise mean value formation of the difference value can be performed.

It is particularly preferable here if a predefined value is subtracted from a value of the integral or a value of the sum.

As a result, a maximum limit value for the total difference between the drive current and the comparison value can be defined with little technical outlay, with the result that an error in the motor current detection can be reliably identified.

In a preferred refinement, the second detection unit for detecting the measurement value of the electrical input variable and the determination unit for determining the calculation value of the electrical input variable are connected in each case to a separate checking unit, wherein the checking units are each configured to independently determine the comparison value in order to independently check the detected value of the drive current. The two checking units in this case preferably exchange the measured and calculated data between themselves in order to provide the respective other checking unit with the required data.

As a result, the drive current can be checked independently, as a result of which the reliability of the checking of the drive current can be increased further.

In a preferred refinement, the separate checking units are connected in each case to a switching unit of the electric machine in order to interrupt the electrical power supply of the electric drive. In this case, the switching units of the electric machine are formed separately and can be actuated independently of one another.

As a result, in the event of an error in the motor current detection, the electric machine or the electric drive can be reliably and independently switched off.

Overall, reliable checking of the measurement of the drive current can be carried out by the present invention with little technical outlay, wherein the thus measured and checked drive current can be supplied to a central control unit with little technical outlay, since the checking takes places separately from the measurement of the drive current and the electrical input variable can be detected externally at the electrical power supply of the electric machine. The different electrical energies in the form of an energy conservation observation can be taken into account by means of the performance model and the physical variables of the individual energy components can be determined separately, with the result that the electrical input variable can be precisely calculated as reference.

It goes without saying that the aforementioned features and those yet to be explained below can be used not only in the combination specified in each case but also in other combinations or on their own, without departing from the scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention are illustrated in the drawing and will be explained in greater detail in the following description. In the drawing:

FIG. 1 shows a schematic illustration of an electric machine having a control unit for driving an electric drive and having an apparatus for checking an electrical variable of the electric machine,

FIG. 2 shows a schematic detail drawing of an embodiment of the electric machine from FIG. 1,

FIG. 3 shows a schematic flow chart of a method for checking an electrical variable of the electric machine, and

FIG. 4 shows a schematic illustration of an embodiment of a coordinate measuring machine as an exemplary application for the electric machine from FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 schematically illustrates an electric machine, which is generally denoted by 10. The electric machine 10 has an electric drive 12 and a control unit 14 for actuating or for energizing the electric drive 12. The electric drive 12 can generally be configured as a rotary motor or as a linear motor and generally has a fixed stator and a moving rotor, wherein, depending on the embodiment, the rotor provides a torque or a linear motor force. The control unit 14 is electrically connected to the stator of the electric drive 12 and actuates the electric drive 12 or energizes the electric drive 12 by means of a motor current I_(A). The torque provided by the rotor or the motor force provided by the rotor is proportional to the motor current I_(A), with the result that the torque or the motor force can be controlled or limited accordingly by way of the motor current I_(A).

The control unit 14 is connected to electrical input connections 16, 18 of the electric machine 10 in order to electrically connect the control unit 14 to an external voltage source 20, which supplies an input voltage or a supply voltage U_(V) to the input connections 16, 18. Depending on the drive power of the electric drive 12, the control unit 14 consumes an input current I_(V), which forms a total consumed current I_(V) of the electric machine 10. A measurement resistor 22 having an electrical resistance R_(S) is arranged in an input line, which connects one of the electrical connections 16, 18 to the control unit 14, wherein a measurement voltage U_(S) is dropped across the measurement resistor 22, said measurement voltage being detected by means of a voltmeter 24 in order to determine the total consumed current I_(V).

An advancing force F of the rotor of the electric drive 12 or a torque M of the rotor is directly proportional to the motor current I_(A) by way of a motor force constant or a torque constant. The motor current I_(A) can be measured or detected by the control unit 14 or a separate ammeter and is available as a non-confirmed measurement value. If the advancing force F or the torque M are intended to be reliably determined or reliably limited with the aid of the motor current I_(A), the measurement of the motor current I_(A) has to be correspondingly confirmed by a further measurement, as is explained in more detail below.

The electric drive 12 can generally be configured as an electric linear motor, as a mechanically commutated DC motor or as a brushless electrically commutated DC motor (BLDC).

To confirm the detected motor current I_(A), the electric machine 10 has an apparatus 30 for checking the motor current I_(A) as the electrical variable of the electric machine 10. The apparatus 30 for checking the motor current I_(A) has a determination unit 32, which determines a calculation value U_(C) for the measurement voltage U_(S) on the basis of the detected motor current I_(A), of the input voltage U_(V) and of a movement variable v, n of the rotor and on the basis of a performance model of the electric machine 10. The apparatus 30 for checking the motor current I_(A) further has a checking unit 34, which compares the calculation value U_(C) for the measurement voltage U_(S) and the measurement value of the measurement voltage U_(S) in order to check the detected value of the drive current I_(A). The checking unit 34 outputs a comparison value between the calculation value U_(C) and the measurement voltage U_(S) and can, on the basis of the comparison value, switch off the electric machine 10 in general or switch off the control unit 14, as is illustrated in FIG. 1.

The apparatus 30 for checking the motor current I_(A) can be configured as an integrated circuit in a chip in the form of a microcontroller.

Since the determination unit 32 determines the calculation value U_(C) from the motor current I_(A) and from the input voltage U_(V) and the movement variable v, n of the rotor, that is to say on the basis of measurement variables that are independent of the measured measurement voltage U_(S), the calculation value U_(C) is independent of the measurement voltage U_(S), with the result that two redundant measurement variables can be compared and the measurement of the motor current I_(A) can therefore be confirmed.

The performance model of the electric machine 10 is based on the principle of the conservation of energy, in that an electrical input power P_(V) of the electric machine 10 is identical to a sum of an output mechanical drive power P_(mech), a power loss P_(M) of the motor and a power loss P_(E) of the electronic components of the electric machine 10, as is shown in formula 1:

P _(V) =P _(mech) +P _(M) +P _(E)   (formula 1)

The electrical input power P_(V) can be calculated with the aid of the measurement resistor 22 and the input voltage U_(V) according to formula 2:

P _(V) =U _(V) *I _(V)   (formula 2)

Alternatively, the input voltage U_(V) and the total current I_(V) can be detected by way of a central control unit in the region of the input connections 16, 18. Furthermore, the total current I_(V) or the measurement voltage U_(S) can be determined by a separate measurement unit at any position of the electrical connection line of the control unit 14, said separate measurement unit supplying the corresponding measurement results to the central control unit by means of a digital bus line.

The electrical power loss P_(E) or the self-consumption power P_(E) of the electronic components of the electric machine 10, such as of the control unit 14, for example, can be measured in advance when the rotor is stationary, wherein the total consumed current I_(V) is then identical to the electric current I_(E) consumed by the electronic components. The electrical power loss P_(E) of the electronic components results from formula 3:

P _(E) =U _(V) *I _(E)   (formula 3)

The power loss of the motor P_(M) results from the motor current I_(A) that flows through the rotor or an armature of the rotor, wherein the armature of the rotor has an armature resistance R_(A). The power loss of the motor P_(M) results from formula 4:

P _(M) =R _(A) ·I _(A) ²   (formula 4)

wherein the armature resistance R_(A) is a known motor constant and the motor current I_(A) can be measured by the control unit 14 or an associated ammeter, as explained above.

In addition to the power loss of the motor P_(M) and the power loss P_(E) of the electronic components, the actual motor power or the mechanical advancing power P_(Mech) also has to be taken into account, which consists of a component proportional to the speed of rotation or speed and of a component proportional to the torque or to the force. The component U_(EMK) proportional to the speed of rotation or speed corresponds to the electromotive force (back EMF) and can be calculated as a movement variable of the electric drive 12 using a motor constant K_(EMK), which is known or can be measured for each electric drive, and the speed v of the rotor or a speed of rotation n of the rotor. The maximum speed of an electric drive 12 results from the fact that the electromagnetic counter-voltage U_(EMK) reaches a value of the maximum available voltage, that is to say the link circuit voltage.

U _(EMK) =K _(EMK) *v; U _(EMK) =K _(EMK) *n   (formula 5)

The component proportional to the torque or to the force corresponds to the motor current I_(A). The actual motor power or the mechanical advancing power P_(Mech) therefore results from formula 6:

P _(mech) =U _(EMK) *I _(A)   (formula 6)

The following thus results for the performance model from formula 1:

U _(V) *I _(V) =U _(V) *I _(E) +R _(A) *I ² _(A) K _(EMK) *v*I _(A)   (formula 7)

For the static case, where the rotor is stationary and therefore no mechanical power is output, the following results for the motor current:

I _(A)√{square root over (U _(V)·(I _(V) −I _(E))/R _(A))}  (formula 8)

and for the dynamic case, taking into account the mechanically output power P_(Mech), the following results for the total current I_(V):

I _(V)=(U _(V) ·I _(E) +R _(A) ·I ² _(A) +K _(EMK) ·v·I _(A))/U _(V)   (formula 9)

wherein the calculation value U_(C) can be calculated by way of the resistance R_(S) of the measurement resistor 22:

U _(C) =R _(S) *I _(V)   (formula 10)

The measured motor current I_(A) can be independently or redundantly checked by way of the comparison value between the calculation value U_(C) and the measurement voltage U_(S), since the calculation value U_(C) is based on the motor current I_(A) and independent variables and the measurement voltage U_(S) can be detected metrologically at the measurement resistor 22. The simple measurement of the motor current I_(A) can therefore be confirmed by way of the performance model and can be used for safety-relevant applications and variables, such as limiting the advancing force, for example.

If the difference value between the calculation value U_(C) and the measurement voltage U_(S) exceeds a predefined value, the measurement of the motor current I_(A) is erroneous, with the result that the measured motor current I_(A) can no longer be used to confirm the advancing force or to confirm the drive torque. In this case, the electric machine 10 can generally be switched off by way of the control unit 14 or a separate switch or contactor.

Since usually smaller differences often briefly arise between the calculation value U_(C) and the measurement voltage U_(S), the difference value can be averaged and the difference value can be summed or integrated for the purpose of identifying an error. Furthermore, a predefined value can be subtracted regularly or continuously from the calculated sum value or calculated integral value, with the result that an error in the motor current detection is identified only after reaching a limit value.

Alternatively thereto, it is also possible to form a moving average value with a tolerance range or the like.

It goes without saying that the total current I_(V) can also be measured by way of the measurement resistor 22, provided a plurality of control units 14 are connected via the measurement resistor 22 to the input connection 16.

FIG. 2 schematically illustrates an embodiment of the electric machine 10. Identical elements are denoted by identical reference numerals, with only the special features being explained here.

In this embodiment, the electric drive 12 is configured as an electrically commutated DC machine that has three phases U, V, W. The control unit 14 is configured as a three-phase inverter, which has three half-bridges each having two controllable switches, in order to convert the input voltage U_(V) to a three-phase AC voltage. The control unit 14 is connected to the three phases of the electric drive 12 by way of three connection lines and accordingly supplies three phase currents I₁, I₂, I₃, in order to drive the rotor of the electric drive 12.

The electric machine 10 furthermore has a first checking unit 36 and a second checking unit 38, which are configured separately and are each preferably configured as an integrated circuit in the form of a microcontroller. The first checking unit 36 and the second checking unit 38 together form the apparatus 30.

The measurement voltage U_(S) and the input voltage U_(V) are supplied to the first checking unit 36 as measured input variables. The input voltage U_(V), the resistance R_(S) of the measurement resistor 22 and the measurement voltage U_(S) as input variables from the first checking unit are supplied as input variables to the second checking unit 38. Furthermore, the second checking unit receives measurement values of the three phase currents I₁, I₂, I₃ and the speed of rotation n and the predetermined current consumption I_(E) as further input variables. The second checking unit 38 calculates the total current I_(V) as explained above with reference to formula 9 on the basis of the input variables and furthermore calculates the calculation value U_(C) by means of the resistance value R_(S) of the measurement resistor 22 and compares the measured measurement voltage U_(S) with the thus determined calculation value U_(C). The second checking unit 38 is connected to the control unit 14 by way of a signal line 42. If the difference value between the calculation value U_(C) and the measured measurement voltage U_(S) exceeds a predefined threshold value, a corresponding switch-off signal is transmitted to the control unit 14 via the signal line 42 and the controllable switches of the half-bridges are opened, in order to disconnect the electric drive 12.

The second checking unit 38 transmits the calculated total current I_(V) to the first checking unit 36 and the first checking unit 36 determines the calculation value U_(C) with reference to the resistance value R_(S) of the measurement resistor 22 and compares the calculation value U_(C) with the measured measurement voltage U_(S) independently of the second checking unit 38. The first checking unit is connected to a switch 35 in the connection line by way of a signal line 40. If the difference value between the measurement voltage U_(S) and the calculation value U_(C) exceeds a predefined threshold value, a corresponding switch-off signal is transmitted to the controllable switch 35 and the controllable switch 35 is accordingly opened.

The first checking unit 36 and the second checking unit 38 are preferably physically separated from one another and exchange the calculated and measured data by way of a bus system 44 or a security bus system 44, as illustrated in FIG. 2. Further redundancy can be established by the physical separation, with the result that, in the event of an error, there are two independent evaluations and switch-off means for the electric drive 12.

FIG. 3 shows a schematic flow chart of a method for checking an electrical variable of the electric machine 10. In FIG. 3, the method is generally denoted by 50.

The method 50 begins with the measurement of the motor current I_(A), as is shown in step 52, wherein the measurement of the motor current I_(A) as explained above has to be confirmed for safety-relevant applications. In step 54, the measurement voltage U_(S) across the measurement resistor 22 is measured and the total current I_(V) is determined. In step 56, the input voltage U_(V) is measured or, in the event of the input voltage being stable and constant, is supplied. In step 58, the movement variable of the rotor is detected, said movement variable being the rotor speed v or the speed of rotation n depending on the embodiment of the electric drive 12.

In step 60, the calculation value U_(C) is determined on the basis of the performance model of the electric machine 10. For calculating the calculation value U_(C), motor constants such as KEMK and R_(A) are provided by a storage means 62 and the previously measured current consumption I_(E) is provided, as is shown in 64. In step 66, the calculation value U_(C) is compared with the measured measurement voltage U_(S). In step 68, the thus determined comparison value is compared with a threshold value and, if the comparison value is greater than the threshold value, in step 70, the electric machine 10 is isolated from the electrical power supply or the electric drive 12 is disconnected. If the comparison value is lower than the predefined threshold value, the method 50 returns to step 52, as is shown by the feedback loop 72, and detects the motor current I_(A).

It goes without saying that steps 52 to 58 can also be carried out in parallel or in any other desired order and that the method 50 can proceed continuously. FIG. 4 illustrates a coordinate measuring machine as an exemplary application for the electric machine 10, said coordinate measuring machine generally being denoted by 80. The coordinate measuring machine 80 serves to measure a workpiece 82 and has a tactile sensor 84 for measuring the workpiece 82. The coordinate measuring machine 80 comprises a carrier structure 86, which can be moved relative to a base plate 88 in the Y-direction. The carrier structure 86 furthermore has a slide 90 and a sleeve 92, which are each mounted in a movable manner and can be moved by means of electric drives. The carrier structure 86 and the slide 90 and the sleeve 92 can be driven by means of an electric machine 10 in accordance with the present invention. The drive forces of the individual elements can be safely limited by confirming the motor current I_(A). 

What is claimed is:
 1. A method for checking a drive current of an electric machine of a coordinate measurement device, wherein the electric machine comprises an electric drive having a stator and a rotor, the method comprising the steps of: detecting a value of the drive current, which is fed to the electric drive for the purpose of driving the rotor, detecting a measurement value of an electrical input variable of the electric machine, wherein the electrical input variable is a total current consumed by the electric machine or an electrical variable corresponding to the total current consumed by the electric machine, determining a calculation value on the basis of the detected value of the drive current and of on the basis a performance model of the electric machine, and determining a comparison value on the basis of the detected measurement value and of the determined calculation value of the electrical input variable, in order to check the detected value of the drive current, wherein the calculation value is a calculated value of the electrical input variable, wherein the comparison value is determined on the basis of a difference value between the measurement value and the calculation value of the electrical input variable.
 2. The method of claim 1, wherein the calculation value of the electrical input variable is calculated on the basis of a total voltage supplied to the electric machine.
 3. The method of claim 1, wherein the performance model takes an electromagnetic force of the electric drive into account.
 4. The method of claim 3, wherein a movement variable of the rotor is detected and wherein the electromagnetic force is determined on the basis of a characteristic variable of the electric drive and of the detected movement variable.
 5. The method of claim 1, wherein the electric machine comprises a control unit, and wherein the performance model takes into account an electrical power loss of the control unit.
 6. The method of claim 5, wherein the electrical power loss of the control unit is determined when the rotor is stationary.
 7. The method of claim 1, wherein the performance model takes into account a mechanical drive power of the electric drive.
 8. The method of claim 1, wherein the comparison value is determined on the basis of a sum of a plurality of the difference values.
 9. The method of claim 1, wherein the comparison value is determined on the basis of an integral of the difference value.
 10. The method of claim 8, wherein a predefined value is subtracted from a value of the integral or a value of the sum.
 11. The method of claim 1, wherein the drive current to the electric drive is interrupted, if the comparison value exceeds a predefined threshold value.
 12. A method for checking a drive current supplied to an electric machine of a coordinate measurement device, wherein the electric machine comprises an electric drive having a stator and a rotor and wherein the electric machine obeys a fundamental principle of conservation of energy, the method comprising the steps of: detecting a first measurement value, said first measurement value representing an instantaneous value of the drive current, while the drive current is fed to the electric drive for the purpose of driving the rotor, detecting a second measurement value, said second measurement value representing a total current consumed by the electric machine, determining a calculation value on the basis of the first measurement value and on the basis of a performance model of the electric machine, said performance model modeling consumed power and output power of the electric machine taking into account the fundamental principle of conservation of energy, and comparing the second measurement value and the calculation value in order to check the instantaneous value of the drive current, wherein the calculation value is a calculated value of the total current consumed by the electric drive.
 13. An apparatus for checking a drive current of an electric machine of a coordinate measurement device, wherein the electric machine comprises an electric drive having a stator and a rotor, the apparatus comprising: a first detection unit, which is associated with the electric drive and is configured to detect a value of the drive current, which is fed to the electric drive for the purpose of driving the rotor, a second detection unit, which is configured to detect a measurement value of an electrical input variable of the electric machine, wherein the electrical input variable is a total current consumed by the electric machine or an electrical variable corresponding to the total current consumed by the electric machine, a determination unit, which is configured to determine a calculation value on the basis of the detected drive current and on the basis of a performance model of the electric machine, and a checking unit, which is configured to determine a comparison value on the basis of the detected measurement value and of the determined calculation value of the electrical input variable, in order to check the detected value of the drive current, wherein the calculation value is a calculated value of the electrical input variable, wherein the checking unit determines the comparison value on the basis of a difference value between the measurement value and the calculation value of the electrical input variable.
 14. The apparatus of claim 13, further comprising at least two separate checking units, wherein the second detection unit and the determination unit each are connected to the at least two separate checking units, wherein the at least two separate checking units each are configured to independently determine the comparison value in order to independently check the detected value of the drive current.
 15. The apparatus of claim 14, further comprising a switching unit configured to interrupt an electrical power supply path to the electric drive, wherein the at least two separate checking units each are connected to the switching unit in order to interrupt the drive current to the electric drive. 